Resistor spark plugs employ a glassy, relatively high resistance seal material between the terminal screw and the center electrode. During spark plug manufacture, such a seal composition, in particulate mixture form, is loaded into the center bore of an insulator body onto the upper end of a previously placed center electrode. A metal terminal screw is then placed in the bore of the insulator so that the lower end of the screw rests on top of the particulate mixture. The assembly is then fired in a furnace at a relatively high temperature to fuse the glass and soften the material so that the terminal screw can be pushed down into the fused composition.
The firing of the composition produces a fused glassy mass, which provides a gas-tight seal in the interior of the spark plug insulator body between the center electrode and the terminal screw. The composition contains metal particles, which, during the firing operation, fuse and provide a bond between the metal conductors and the resistive seal composition.
Spark plug resistor glass seals have proven to be effective in suppressing high frequency oscillations that occur during spark discharge in an automotive ignition system. These oscillations lead to electromagnetic interference that can effect radios, computers and other electronic automotive components. In performing this function, it is important that the original particulate mixture fuse upon firing to form a mass that has a predictably high level of resistance and that such level of resistance not change appreciably during prolonged usage of the spark plug in engine operation.
The prior art teaches two basic types of monolithic spark plug resistor glass seals, both of which are described in U.S. Pat. No. 3,567,658. One type of monolithic spark plug, disclosed in U.S. Pat. No. 2,864,884, has a resistive glass seal comprising a semi-conductor material, glass, filler material, and a small percentage of a reducing agent, such as powdered metal or carbon, to control the resistivity of the seal. Examples of semi-conductor materials in this type of monolithic spark plug are TiO.sub.2, SnO.sub.2, Ta.sub.2 O.sub.5, MoO.sub.3 and Al.sub.2 O.sub.3.
A second type of monolithic spark plug, such as disclosed in U.S. Pat. No. 2,459,282, has a resistor glass seal comprising a heterogeneous mixture of conductive materials, such as carbon, metals, metal oxides, and metal carbides, dispersed within a continuous glass phase. In such spark plugs, the resistance of the plug is dependent on the concentration of the conductive phase while the glass forms a dense, hermetic seal. Due to the heterogeneous nature of the conductive phase and the multiple conductive materials therein, part-to-part variation is often difficult to minimize. Historically, carbon based materials have been added to the mixture in forms which range in size, density and degree of water solubility. Such differences in the raw materials lead to variation in the continuity of the carbon phase and difficulty in reproducing the overall spark plug resistance. Processing of the raw materials so as to maintain microstructural uniformity becomes a significant challenge.
Most notably, the prior art makes use of a conductive phase consisting of a water soluble form of carbon (e.g. 10-X sucrose) and a solid particulate form of carbon (e.g. thermax or graphite). These two materials must be added carefully to control the resistivity of the final composite material. Because both materials yield a conductive form of carbon after significant heat treatment, the corresponding resistance of the spark plug decreases as the concentration of the carbon sources increases. Also, the ratio of the 10-X sucrose to the thermax is important. When acting alone, the 10-X sucrose causes an increase in resistance due to electrical aging of the glass seal. Conversely, the thermax, when acting alone, will cause a decrease in the resistance of the spark plug due to electrical aging. A ratio of the two materials can be determined which yields minimal electrical aging during the lifetime of the plug. Thus, a proper concentration and ratio of each carbon source is necessary for a satisfactory glass seal.
Another important consideration is the distribution of the two carbon sources within the glass seal. Typically, the particulate carbon is poorly distributed in the glass seal body and can be detected as clusters rich in carbon. Compositional homogeneity of the resistor seal is necessary for a seal with very tight resistance tolerance. Homogeneous mixing of the thermax is difficult due to its low concentration in the mixture (less that 3% by weight) and its relatively hydrophobic nature. Thus, high energy agitation is necessary to ensure sufficient mixing.
Another difficulty arises with the migration of the 10-X sucrose within the mixture, as water is evaporated during drying. As water migrates to the surface of the particles where it evaporates, the 10-X sucrose can migrate as well, resulting in uneven distribution and an undesirable conductive coating on the outside of the agglomerates. Both phenomena lead to non-uniformity in the composition and variability in the resistivity of the material.
Another shortcoming of the prior art relates to spark plug failure attributable to a breakdown of the bonding between resistor glass seals and spark plug electrodes. Due to extreme electrical and thermal stress in the spark plug during operation, the interface of the seals and electrodes is often disrupted, leading to operation failure. To account for this, spark plugs with conductive glass seals, in addition to the resistor glass seal, are currently in use. The conductive glass seals typically consist of metal particles, commonly copper or nickel, dispersed within a glass phase. The conductive seals provide for a low electrical resistance path from the center electrode or the terminal post surface to the resistor glass seal interface. The conductive seals are typically positioned on each end of the resistor seal to directly interface with the electrodes. The primary disadvantage of adding conductor seals is the resulting reduction in length of the resistor seal. The conductor seal is placed into the plug at the expense of the resistor, shortening the ends of the resistor seal. This change in the length of the resistor seal decreases its ability to suppress radio frequency interference (RFI), its primary function.
Efforts have been made to improve bonding of the resistor glass seal to the metal electrodes by manipulating the formulation of the glass. While some advancements have been made, they have not proven beneficial for the current application. Thus, a strong need exists for a resistor glass seal material that will bond effectively to the metal electrodes in the spark plug.